1. Field of the Invention
The present invention relates to a lens assembly equipped with an image-forming optical system and to an image-taking apparatus provided with the lens assembly.
2. Description of the Related Art
In recent years, automobiles have been provided with cameras. Meanwhile, nowadays, automobiles are equipped with display screens thanks to the widespread use of car navigation systems. Therefore, many of cameras installed in automobiles can display the state of a blind spot for a driver on a display screen provided in front of the driver's seat.
Incidentally, automobiles may be left outdoors in hot summer and in freezing winter and therefore, a camera installed in an automobile is required to operate properly over an extremely wide range of temperatures. In order to guarantee the proper operation of a vehicle-mounted camera, a lens assembly employed in such a camera needs to be not only lightweight and robust as generally required, but also resistant to stretching and shrinking with temperature for the purpose of preventing the occurrence of a blur due to a change in temperature. To meet these needs, ceramic barrels have been devised (see, for example, Japanese Patent Application Publications No. 2006-284991 and No. 2006-292927). Also, the inventors of the present application have proposed to apply porous ceramic to an optical component (see, for example, Japanese Patent Application Publication No. 2007-238430). The porosity of the porous body described in the specification of Japanese Patent Application Publication No. 2007-238430 is equal to or more than 10%.
However, the porous ceramic is prone to chip, which is a drawback that needs to be overcome.
Here, how such a drawback appears when the porous ceramic is used in a barrel will be described with reference to FIG. 1.
FIG. 1 is a diagram showing an example of the structure of a lens assembly to be applied to a vehicle-mounted camera, and FIG. 2 is a diagram for describing the state of a defect that has occurred at the time of assembling the lens assembly. Further, FIG. 3 is a diagram for describing the structure of a positioning section different from a positioning section shown in FIG. 1.
First, the structure of the lens assembly will be briefly described with reference to FIG. 1.
A lens assembly 1 illustrated in FIG. 1 is provided with a barrel 10, and the barrel 10 has a hollow part 100 having an object-side opening 101 and an image-forming-side opening 102. Formed on an object-side periphery of the barrel 10 is a male thread SR1. From the object-side opening 101, lenses L1 trough L4 and spacing rings SP1 through SP3 are inserted while being aligned along the optical axis. These elements L1 through L4 and SP1 through SP3 may be collectively referred to as optical members. In this example, as shown in FIG. 1, the lenses L1 trough L4 and the spacing rings SP1 through SP3 are alternately disposed and sequentially inserted into the hollow part 100 of the lens barrel 10.
Furthermore, the lens assembly 1 illustrated in FIG. 1 is provided with a pressing ring 11 that fixes the optical members L1 through L4 and SP1 through SP3 inserted into the hollow part 100 of the barrel 10, by pressing these optical members from the side where the object-side opening 101 is formed. The pressing ring 11 has: a mounting opening 110 into which an object-side part of the barrel 10 is inserted; and an optical opening 111 for making a central part of the lens L1 exposed. Among the optical members inserted into the barrel, the lens L1 is the one disposed at the position closest to the object. Formed on an inner wall on a mounting opening side of the pressing ring 11 is a female thread SR2 in which the male thread SR1 is engaged. When the male thread SR1 is engaged in the female thread SR2, the edge of an object-side surface of the lens L1 disposed at the position closest to the object on the object side is pressed by the pressing ring 11.
The lens assembly 1 shown in FIG. 1 is assembled by using the pressing ring 11 that presses the lenses L1 trough L4 and the spacing rings SP1 through SP3 in the barrel 10 toward the image-forming-side opening.
In the example shown in FIG. 1, the positioning section 103 positions an image-forming-side surface of the lens L4 disposed at the outermost position on the image-forming side, by having line contact around the optical axis at a center. The positioning section 103 is provided at a position in which the image-forming-side opening 102 is formed, so that the positioning section 103 can serve as a stopper for eliminating unwanted light. In other words, the positioning section 103 forms the edge of the image-forming-side opening 102. For this reason, the positioning section 103 is formed to have an acute angle in cross section including the optical axis, as illustrated in part (b) of FIG. 1.
Thanks to this structure, after the lens L4 located at the outermost position on the image-forming side is accurately positioned by the positioning section 103, the remaining lenses L1 through L3 are sequentially positioned by following the positioned lens L4. However, since the porous ceramic has such a drawback that it is liable to chipping as described above, the positioning section 103 illustrated in FIG. 1 may chip as well during assembly. Once such chipping occurs, not only the commercial value in view of the appearance is lowered, but also the quality of a shot image is reduced because incident light diffuses at a chip and a part of the incident light enters the shooting angle. In addition, such a chip may cause the lens L4 to lean without being precisely positioned, impairing the image.
FIG. 2 illustrates the state of a defect that has occurred when the porous ceramic is used for the barrel, and shows the state where many chips 1031 have appeared in the positioning section with the acute angle. In order to prevent these chips 1031 from being formed, it is conceivable to reinforce the positioning section. For example, a thick positioning section 103A can be formed by utilizing the technique of Japanese Patent Application Publication No. 2007-279557 as illustrated in FIG. 3. However, the positioning section 103A has such a drawback that subject light reflects at a portion 1031A and enters a shooting angle thereby lowering the quality of a shot image as in the case in which a chip is formed. For this reason, it is necessary to make the positioning section have an acute angle like the positioning section 103 illustrated in FIG. 1, while preventing a chip.
The drawback of the porous ceramic barrel has been described so far. However, even when a barrel is produced using ordinary non-porous ceramic, chipping may similarly occur, although this barrel may be stronger than the porous ceramic barrel. Alternatively, it is conceivable to employ a barrel made of, for example, plastic, by designing the barrel that can resist stretching and shrinking with temperature. The barrel made of plastic is free from a problem of a chip unlike the ceramic barrel. Instead however, the barrel made of plastic has such a drawback that when it is used as a material of the positioning section 103 illustrated in FIG. 1, the positioning section 103 is deformed when pressed by the lens L4, shifting the positions of the lens L4 as well as other lenses, thereby failing to realize the initial optical performance.